This invention relates generally to the design configuration of supersonic aircraft with wings designed for extensive natural laminar flow (NLF), and more particularly to optimization of wing thickness and fuselage cross section relationship criteria, for such supersonic aircraft.
Supersonic natural laminar flow wing configurations are desirable for efficient supersonic cruise. Principal features are low sweep, sharp (actually very thin) leading edge, and thin biconvex-type airfoils offering a supersonic cruise drag advantage as a result of reduced skin friction drag associated with NLF, which more than offsets increased drag due to thickness (volume wave drag). The importance of laminar boundary layer (BL) flow in terms of drag reduction can be seen from the fact that for typical supersonic cruise flight conditions the laminar skin friction drag is approximately a factor of ten less than turbulent skin friction drag associated with traditional supersonic swept or delta wing, for the same amount of surface area. The NLF wing also provides additional advantages over traditional supersonic swept or delta wings in cruise efficiency at high subsonic speeds, and in takeoff and landing performance. In addition the NLF wing can achieve its best efficiency at a substantially higher subsonic Mach number than that of the swept wings typically used on high speed subsonic aircraft.
The supersonic NLF wing must have low sweep and therefore incurs more volume wave drag (related to thickness) than a well-designed delta wing of similar size and thickness.
Thus, on a purely aerodynamic basis the low sweep NLF wing should be as thin as possible, to reduce the volume wave drag. On the other hand a thinner wing incurs a weight penalty, since structural weight increases as wing thickness decreases, so that selection of thickness to chord ratio (t/c) is a key to optimizing the performance of such aircraft.
In our previous design studies, the wing was limited to thickness-chord ratios (t/c) for which the volume wave drag was appreciably less than the drag savings resulting from laminar skin friction vs turbulent skin friction. This consideration formed certain bases for U.S. Pat. Nos. 5,322,242, 5,518,204, 5,897,076, “High Efficiency Supersonic Aircraft”, incorporated herein by reference. As will be shown, this criterion led to the selection of about 20 (0.02) as an upper limit for the average t/c of the wing, for the Mach 1.5 speed then being considered. As mentioned, these prior patents claimed a t/c less than about 2%, but specified no variation with design cruise Mach number, M. The curve of FIG. 6 is representative of that variation and can be approximated by,
      t    c    ≤      0.02    ×                            M          -          0.5                    .      
Nonetheless, a number of considerations drive the optimal thickness to higher values, even at the expense of more volume wave drag. For example the favorable pressure gradient, which stabilizes the laminar boundary layer, increases with wing t/c, and as noted, structural weight decreases with increasing thickness. In addition, the volume wave drag attributable to the wing can be reduced by contouring the fuselage in the vicinity of the wing. Finally, the achievement of NLF on large areas of the wing surface is dependent on (a) achieving appropriate pressure gradients over the affected surfaces of the wing and (b) suitable leading edge size and shape. These pressure gradients depend not only on the local airfoil shapes, but also are significantly influenced at supersonic speeds by the fuselage contours adjacent to, and forward of the wing. There is, accordingly, need for improvements in such aircraft, and particularly in the optimization of the biconvex-type airfoil shape and thickness, as well as the fuselage contours affecting both volume wave drag and NLF extent over the wing surfaces.